This invention relates to loudspeakers, and, more particularly, to a loudspeaker having an upper diaphragm and a lower diaphragm which provide extended and substantially improved range of motion in response to movement of the voice coil without bending, buckling, twisting or other deformation which can create acoustic distortion.
Loudspeakers generally comprise a frame, a motor structure, a diaphragm, an upper suspension or surround and a lower suspension or spider. In one common type of speaker, the motor structure includes a permanent magnet mounted between a top plate and a back plate, a pole piece centrally mounted on the back plate and a voice coil axially movable with respect to the pole piece. The voice coil includes a hollow, cylindrical-shaped former having an outer surface which receives a winding of wire.
One end of the diaphragm is connected to the surround, which, in turn, is mounted to the upper end of the frame. The spider is connected at one end to a seat formed in the frame at a point between its upper and lower ends. The free ends of the diaphragm and spider are mounted to the voice coil so that it is supported within the magnetic gap between the pole piece and top plate of the motor structure, with the former of the voice coil being concentrically disposed about the pole piece. In some speaker designs, a dust cap is mounted to the diaphragm in position to overlie the voice coil and pole piece to protect them from contaminants.
In the course of operation of a speaker of the type described above, electrical energy is supplied to the voice coil causing it to axially move relative to the pole piece and within the magnetic gap formed between the top plate and pole piece. The diaphragm, spider and surround move with the excursion of the voice coil. Conventionally, the diaphragm is formed in the shape of a cone with an opening cut at its apex in order to mount to the outer surface of the former of the voice coil. The diaphragm extends radially outwardly at an angle from the voice coil to the surround where it is typically connected by adhesive or otherwise permanently affixed. The driving force of the voice coil is applied at approximately the center of the diaphragm, and the diameter of the voice coil is typically much smaller than that of the diaphragm. As a result, there is a wide span along the wall of the diaphragm between the voice coil and surround which is unsupported and subject to deformation.
Deformation of the diaphragm can result from a number of factors, some involving the operation of the speaker and others related to speaker construction. In most applications, the speaker is contained within a cabinet or box which is substantially enclosed to provide protection for the speaker. At low frequencies and high axial excursions of the voice coil, the piston-like movement of the voice coil, diaphragm and spider pressurizes the air within the interior of the speaker box. In one direction of movement of the voice coil, spider and diaphragm, the air within the box is compressed thus tending to push the diaphragm in the opposite direction. Movement of the diaphragm in the reverse direction causes the air within the box to exert a pulling force on the diaphragm. These push and pull forces exerted on the diaphragm can lead to bending or buckling of its wall, creating acoustic distortion. Further, at or near the limits of axial movement of the voice coil and diaphragm permitted by the upper and lower suspensions, such as when the speaker is operated at very high output levels, substantial forces are applied to the diaphragm at its attach points to the voice coil and to the surround, thus causing an increase in bending forces applied to the diaphragm and creating another source of acoustic distortion.
Additionally, as the driving force exerted on the diaphragm by the voice coil increases in frequency, there is a tendency of the outer edge of the diaphragm located at the surround to lag behind its inner edge connected to the voice coil. This xe2x80x9clagxe2x80x9d is due to the mass of the diaphragm and the natural compliance of the material from which it is constructed. Past a certain frequency threshold, depending upon the stiffness of the diaphragm material, the wall of the diaphragm no longer moves as a single unit thus creating undesirable acoustic peaks and valleys in the reproduced sound field.
Conventional diaphragms are also susceptible to twisting during operating of the speaker, which is yet another source of acoustic distortion. During assembly, the circular-shaped surround may not be correctly centered on the frame causing one side or area of the surround to be somewhat stretched or extended, while another area is compressed to some extent. In response to movement of the voice coil in one direction, the area of the surround which was stretched to some extent during assembly reaches its limit of extension before the area which was compressed. Conversely, a movement of the voice coil in the opposite direction causes the area which was initially compressed to reach its compression limit before the area which was initially stretched. Consequently, the surround tends to tilt or twist with the movement of the voice coil, which, in turn, causes the diaphragm and voice coil to twist in the same fashion. Such twisting forces imposed on the diaphragm are another source of potential acoustic distortion.
Attempts have been made to reinforce the diaphragm of conventional loudspeakers and provide resistance to bending and twisting forces. One approach has involved the use of two diaphragms, including one outer diaphragm connected to the surround, and at least one second, inner diaphragm connected to the voice coil and to the outer diaphragm. See, for example, U.S. Pat. No. 4,764,968 to Dreitmeier; U.S. Pat. No. 4,275,278 to Sakurai et al; and U.S. Pat. No. 4,567,327 and 4,517,416 both to Goossens.
Despite the use of a second diaphragm for reinforcement of the primary, sound producing diaphragm, the designs in the prior art noted above suffer from common limitations. In each case, the inner diaphragm of such designs is formed with an opening at its center to receive and mount to the former of the voice coil. Particularly in high excursion speakers, the former of the voice coil must be relatively long and therefore it is commonly made from lightweight material to reduce mass. This tends to make the voice coil flimsy in the area where the inner diaphragm is connected. The combination of the opening at the center of the inner diaphragm, and its connection to the relatively weak or flimsy voice coil former, substantially reduces the degree of stiffness which can be provided by the inner diaphragm to the outer diaphragm.
Another limitation of prior two diaphragm designs is that no attempt is made to seal the space in between the inner and outer diaphragms in the area above the voice coil, or, in some instances, anywhere between the two diaphragms. The absence of a seal between the diaphragms results in the alternating pressurization and depressurization of the air in the space between the two diaphragms, which tends to collapse or pull apart the diaphragms depending on the direction of movement of the voice coil. This increases the chance of bending or other deformation of the diaphragms, leading to unwanted distortion.
It is therefore among the objectives of this invention to provide a loudspeaker which provides improved acoustic performance, particularly at low frequency and high excursions, which limits bending and twisting of the diaphragm and which resists forces imposed by pressurization of the enclosure within which the loudspeaker is mounted.
These objectives are accomplished in a loudspeaker comprising a motor structure including a voice coil with a hollow former, a frame having an upper end and a lower end connected to the motor structure, an upper suspension mounted to the upper end of the frame, and, a two part diaphragm structure which includes an upper diaphragm connected to the upper suspension and a lower diaphragm extending between the upper suspension and the voice coil in position to overlie the hollow former of the voice coil.
In the presently preferred embodiment, the lower diaphragm is a formed in a one-piece construction which extends continuously from the upper suspension to a position overlying the voice coil. No opening or hole is formed in the lower diaphragm to mount it to the former of the voice coil, as in the prior designs described above, but instead the lower diaphragm is integrally formed with a protrusion which either mounts directly to the exterior surface of the former of the voice coil or to a sleeve connected to the voice coil. In either case, the stiffness of the lower diaphragm is enhanced without the formation of an opening therein, which substantially increases the overall stiffness of the inner and outer diaphragm construction compared to the prior art. This is particularly true for high excursion loudspeakers in which the former is comparatively long to accommodate such excursions and therefore must be formed of a lightweight and relatively flimsy material to decrease mass. The continuous lower diaphragm adds stiffness to the former, and, hence, the entire moving assembly of the speaker, which is not achieved by the connection between the former and a lower diaphragm having a hole in the center as in other designs.
In the embodiment of this invention employing the sleeve noted above, the outer diaphragm includes an outer section connected to the outer suspension, and an inner section having a cross section in the shape of a truncated cone with a generally planar top wall and a tapered side wall. The protrusion is formed at the juncture of the tapered side wall of the inner section and the outer section of the lower diaphragm. Preferably, the top wall of the inner diaphragm is mounted to the upper diaphragm by adhesive or the like, which further enhances the overall stiffness and rigidity of the combined upper and lower diaphragm structure. The protrusion of the lower diaphragm is affixed within a trough or well formed at the upper end of the sleeve during the assembly process.
Alternatively, the sleeve is eliminated as noted above, and the lower diaphragm is mounted directly to the former of the voice coil. In this embodiment, the lower diaphragm has an outer section mounted to the upper suspension, and an inner section having a generally semi-circular cross sectional shape positioned to overlie the hollow interior of the former. The protrusion is formed at the juncture of the inner and outer sections of the lower diaphragm, which includes a generally vertically extending wall shaped to rest against the outer surface of the former of the voice coil where it is mounted by adhesive or the like.